Naruto Gekitou Ninja Taisen 4 is the best game for the GC in my opinion ^^ Though you need a Freeloader or Action Replay to play this game since its a Japanese import.
Also I'm seriously addicted to Harvest Moon Magical Melody.

i just picked up some Sonic package for $10 that has the first 3 sonics, Sonic 3d, sonic spinball, some other game that's like tetris, and some games that I unlock somehow. i'm having fun with it until GT3 come out!